Oligoclonal IgG banding and the presence of oligoclonal populations of T cells in the central nervous system (CNS) of patients with multiple sclerosis (MS) remains an unexplained hallmark of the disease. Yet, it is still unknown whether there are specific antigen reactivities of clonally expanded T or B cells in the target organ of patients with the disease. In collaboration with Project 1 of this Program Project Grant, we will employ new technologies to examine fundamental issues related to the role of T cells in patients with MS. First, we will use combinatorial peptide libraries to examine the antigen specificity of oligoclonal CSF T cells. These investigations will also be performed on subjects with defined CNS infections, and will allow us to determine whether T cell receptors expressed on CNS T cells in patients with MS are degenerate in their recognition of combinatorial peptide libraries as compared CNS T cells recognizing viral or bacterial antigens. We will then use tetramer/MHC complexes of DR2 (DR2*1501)/MBp85-99 to both define the frequency and phenotypic function of MBPp85-99 reactive T ells in the CSF and blood. In both investigations we will determine whether the cloned T cells are reflective of the original CSF population as defined by TCR CDR3 length analysis and sequencing. Together, these investigations may elucidate whether the T cells in the CSF compartment of patients with MS are clonally expanded by stimulation with a specific antigen, be it a self or microbial antigen. Lastly, in collaboration with Ploegh in Project 3 of this Program Project Grant, we will define immunodominant epitopes and examine the CNS processing of MOG and MBP, CNS proteins of potential importance in the immune response to myelin in patients with MS. Insights into the cell biology of Class II restricted presentation will be applied to the processing of MOG and MBP using APC derived from peripheral blood and from CNS microglial cells. We will determine whether the same epitopes are presented by circulating monocytes as compared to CNS technologies that may allow us to define the importance of CNS T cells in patients with MS, and to develop new surrogate markers for monitoring patients with MS.